The objective of the proposed research remains to learn more about human ocular motor disorders that impair vision, and to appraise therapeutic measures. The research strategy is to (1) study natural behaviors such as combined eye-head tracking; (2) relate reliable measurements of eye movements to visual complaints of patients; (3) interpret the control of normal and abnormal eye movements using mathematical models that are based on known physiologic properties. In this way we plan to apply new knowledge of pathogenesis to the diagnosis and treatment of disorders of ocular motility. This proposal has five aims that emerge from prior work. One is to evaluate the role of botulinum toxin injections into selected extraocular muscles as a treatment for impaired vision and oscillopsia due to acquired nystagmus. By selecting patients in whom stabilization of retinal images by electronic means improves vision, it should be possible to produce appreciable, if temporary, improvement of symptoms. Two further projects concern study of torsional eye movements, relating them to the visual demands that they serve. We will focus on normal visuo-vestibular interaction, and investigate the mechanisms that account for suppression of vestibular eye movements when subjects view a display that moves in the roll plane with their heads. We will also study torsional eye movements in patients with disorders of vertical or horizontal gaze, to identify the involvement of the torsional system. The aim of a fourth project is to compare dynamic properties of ocular smooth-pursuit eye movements with those of combined eye-head tracking, the latter being the more natural way to visually track a moving object. The aim of the fifth project is to determine how discrete, unilateral lesions of cerebral cortex impair combined eye-head tracking, compared with purely ocular smooth pursuit. These projects will provide (1) an evaluation of a new treatment method for visual symptoms due to acquired nystagmus; (2) new information on the role of torsional eye movements and the deficits produced by disease; (3) new insights into how natural, combined, eye-head tracking allows a clear view of a moving object, and how such tracking is affected by lesions affecting secondary visual areas in cerebral cortex.